The present invention relates to a magnetic recording device and method, and more particularly, to a magnetic recording device and method in which a magnetic head records audio and video information on a magnetic recording medium by using a low power-consuming recording amplifier suited for a high-speed and high-density recording.
Magnetic recording of information is accomplished by generating a magnetic flux by means of a head current flowing through a magnetic head and thus magnetizing a magnetic recording medium. Generally, a recording amplifier of a constant current source is used to supply a current of an intended signal waveform to a magnetic head having an inductive impedance. To obtain the intended current signal waveform, its corresponding voltage signal waveform should be generated in a signal processor, in advance.
FIG. 1 is a circuit diagram of a conventional A-class single-ended recording amplifier.
As shown in the figure, a current feedback resistor R.sub.k is connected to the emitter of an amplifying transistor Q.sub.a. Applied to the base of the transistor Q.sub.a is an input voltage signal E.sub.i and a bias current I.sub.bo and a base current I.sub.br corresponding to the input voltage signal E.sub.i shown in FIG. 2A. The transistor Q.sub.a operates by this base current I.sub.br. Therefore, a collector alternate current (AC) component I.sub.a, i.e., a recording current I.sub.r shown in FIG. 2B, as well as a direct current (DC) component Q.sub.ao corresponding to the bias current I.sub.bo are supplied to the primary part of a rotary transformer (hereinafter referred to as R/T), and a head current I.sub.h shown in FIG. 2D flows in the secondary side of the R/T.
Meanwhile, when a binary coded recording current I.sub.r, indicative of binary coded information, flows and is switched in an inductance load such as the magnetic head, a transient pulse voltage V.sub.tr shown in FIG. 2C is generated across the head, in this case at the collector of the transistor Q.sub.a. To prevent the waveform of a collector voltage V.sub.a from being distorted, the transistor Q.sub.a should function as an A-class amplifier by assigning its operation quiescent point on a linear portion of its characteristic curve.
In the A-class recording amplifier shown in FIG. 1, EQU E.sub.i .perspectiveto.I.sub.a R.sub.k, or I.sub.a /E.sub.i .perspectiveto.constant (1)
The recording amplifier of FIG. 1 can be approximated to an equivalent circuit of FIG. 3, in which a head impedance Z.sub.h being an inductive impedance is generated by connecting in parallel with one another an equivalent inductor L.sub.h, a loss resistor R.sub.h, and an equivalent capacitor C.sub.h. Assuming that an upper limit frequency of a transmission band is f.sub.m and the ratio of the number of windings at the primary side to that of windings at the secondary side of the R/T is N , EQU R.sub.h /(2.pi.f.sub.m L.sub.h).perspectiveto.(3.about.4)&gt;1(2)
Since the load of the transistor Q.sub.a of FIG. 1 is an inductive impedance, an output impedance R.sub.s of the transistor Q.sub.a should be larger than the head impedance Z.sub.h to flow the predetermined head current I.sub.h. ##EQU1##
Given an amplification degree as A, the recording current I.sub.r output from the transistor Q.sub.a is calculated by EQU I.sub.r =A.multidot.E.sub.i /(R.sub.s +Z.sub.h).perspectiveto.A.multidot.E.sub.i /R.sub.s ( 4) EQU Z.sub.h =R.sub.h .multidot.j2.pi.L.sub.h /(R.sub.h +j2.pi.L.sub.h)=j2.pi.L.sub.h ( 5)
Accordingly, a recording-equalization compensation can be performed in the linear amplifier of FIG. 1 for providing the head current I.sub.h proportional to the input voltage signal E.sub.i.
In this case, with the upper limit frequency f.sub.m of an effective band, the condition such that R.sub.s &gt;.vertline.Z.sub.h .vertline. should be satisfied based on the premise that ##EQU2##
FIG. 4 illustrates a B-class push-pull recording amplifier having a pulse transformer (hereinafter referred to as P/T), and FIG. 5 illustrates a B-class push-pull recording amplifier without the P/T.
In the B-class push-pull recording amplifier of FIG. 4, the collector of a transistor Q.sub.k having a current feedback emitter resistor R.sub.k, for controlling a constant current, is connected to a common emitter of transistors Q.sub.a and Q.sub.a ' exhibiting high-power impedance characteristics. Since the recording amplifier is a push-pull type, DC components can be cancelled and, practically, neglected at the primary side of the P/T.
Base currents I.sub.br and I.sub.br ' generated by input voltage signals E.sub.i and E.sub.i ' shown in FIGS. 6A and 6B are injected to the respective bases of the transistors Q.sub.a and Q.sub.a, thus alternately turning on and off the transistors Q.sub.a and Q.sub.a '.
A predetermined recording current I.sub.a shown in FIG. 6C is transferred to the secondary side of the P/T and converted into a recording current I.sub.r, when the transistor Q.sub.a is turned on by the base current I.sub.br. A predetermined collector current Q.sub.a ' shown in FIG. 6D is transferred to the secondary side of the P/T and converted into the recording current I.sub.r, when the transistor Q.sub.a ' is turned on by the base current I.sub.br '. The recording current I.sub.r is supplied to a head H'D through the R/T and thus a head current I.sub.h shown in FIG. 6E flows through the head H'D. Here, switches for a recording/reproducing head are used as recording/reproducing switches REC/PB SW and REC/PB SW'.
On the other hand, as compared with the recording amplifier of FIG. 4, the recording amplifier of FIG. 5 has resistors R.sub.L and R.sub.L ' as collector loads connected to the push-pull amplifiers Q.sub.a and Q.sub.a ', respectively, thus omitting the P/T.
Recording equalization of the B-class push-pull recording amplifier will be described in more detail, with reference to FIG. 4.
An equivalent circuit of the recording amplifier shown in FIG. 4 is illustrated in FIG. 7A. Referring to FIG. 7A, when a head impedance Z.sub.h is measured in a small signal, the recording amplifier of FIG. 4 can be approximated by the equivalent circuit of FIG. 7A by connecting in parallel an equivalent loss resistor R.sub.h, an inductor L.sub.h and a parasitic capacitor C.sub.h, which are surrounded by a dotted block. The same equivalent circuit can be obtained in the case where a large current such as a recording current flows.
A current supplied through respective source output resistors R.sub.s by means of the input binary coded signal E.sub.i and its polarity-reverted signal E.sub.i ', switched in the switches SW and SW', passes through the P/T and R/T, and reaches the magnetic head.
Here, the coupling coefficient of the P/T is nearly 1.00, and that of the R/T is about 0.94-0.98. Thus, the leakage inductance of the P/T is negligible. On the assumption that the leakage inductance of the R/T is L.sub.K, inductances at the primary sides of the P/T and R/T are L.sub.PT and L.sub.RT, respectively, a stray capacitance existing in an actual circuit is C.sub.s, and the ratio of the number of turns of the stator (the primary side) and the rotator (the secondary side) of the R/T is N, the equivalent circuit of FIG. 7A can be simplified to a circuit of FIG. 7B.
The equivalent circuit of FIG. 7B can be further simplified to a circuit shown in FIG. 7C by a primary approximation based on practical conditions such that ##EQU3##
A head exciting current i.sub.L flowing in the head inductor L.sub.h for producing a recording magnetic field can be obtained in a circuit of FIG. 7D which is equivalent to the circuit of FIG. 7C by substituting C.sub.s, N.sup.2 L.sub.h, and N.sup.2 R.sub.h for C, L, and R, respectively. A recording current i.sub.R can be approximated to E.sub.i /R.sub.s, as described above.
A current i.sub.C flowing through a total stray capacitor C at the primary side of the R/T is initially determined, and then a current i.sub.RS flowing through a loss resistor R of the magnetic head and a current i.sub.L flowing through an inductor L of the magnetic head are calculated.
As shown in FIG. 8A, a period .tau. of the i.sub.c waveform is defined as one cycle of a sine wave, and a voltage V.sub.p applied to the capacitor C is calculated by using the cycle .tau.. EQU i.sub.C =i.sub.CO .multidot.sin(2.pi.t/.tau.)=C.multidot.dV.sub.p /dt(6)
where i.sub.CO is a maximum value of i.sub.C and ##EQU4##
The current i.sub.RS of FIG. 8B flowing through the resistor R can be calculated by EQU i.sub.RS =V.sub.p /R=.quadrature.(i.sub.C .multidot.dt)/CR (8)
The ratio of the maximum value i.sub.RO of the current i.sub.RS shown in FIG. 8B to the maximum value i.sub.CO of the current i.sub.C shown in FIG. 8A is given by EQU i.sub.RO /i.sub.CO =.tau./.pi.CR (9)
The waveform of the current i.sub.L flowing through the inductor L is illustrated in FIG. 8C, and EQU V.sub.p =-L.multidot.di.sub.L /dt, thus EQU i.sub.L =-1/L.multidot..quadrature.V.sub.p dt (10)
Therefore, to flow an intended current through the inductor L, it is necessary to supply both the current i.sub.C and the current i.sub.RS as the recording current i.sub.R. Accordingly, EQU i.sub.R =i.sub.L +i.sub.C +i.sub.RS ( 11)
The ratio of the maximum value i.sub.LO of the current i.sub.L shown in FIG. 8C to the maximum value i.sub.CO of the current i.sub.C is expressed as EQU i.sub.LO /i.sub.CO =.tau..sup.2 /(2.pi.LC) (12)
On the other hand, a stray parasitic capacitance on the recording/reproducing switches REC/PB SW and REC/PB SW' or on collector distributing capacitors C.sub.SO and C.sub.SO ' exists in the recording amplifier of FIG. 4. If the switches SW and SW' are semiconductor devices, there exists an additional 10pF of stray capacitance, and the parasitic capacitance of a drum assembly is 10pF or above, including those of the R/T and a flat cable. However, the stray capacitance C.sub.S is generally considered to be approximately 20pF, in total. The sum (i.sub.L +i.sub.C) of the currents i.sub.L and i.sub.C is illustrated in FIG. 8D.
Therefore, as the stray capacitance becomes larger, the rise characteristics (average rise time and .tau.) of the head magnetizing current i.sub.L are degraded, as noted from equation (7). A dotted line in FIG. 8C indicates the waveform of the head magnetizing current i.sub.L exhibiting the degraded rise characteristics.
To obtain i.sub.L having the rise characteristic as indicated by a solid line in FIG. 8C, a charging and discharging current i.sub.c should be provided to a stray capacitor C.sub.s, simultaneously. To achieve a current exhibiting a steeper rise characteristic curve shown in FIG. 8D, it is necessary to improve the rise characteristic of the input voltage signal E.sub.i in the recording amplifier.
Also, to improve the rise characteristic of i.sub.L, the sum current (i.sub.L +i.sub.RS) showing a steeper rise characteristic curve than that of i.sub.L should be supplied as a head magnetizing current. The current (i.sub.L +i.sub.RS) is smaller than that of i.sub.L compensated for by i.sub.C, i.e., a current (i.sub.L +i.sub.C) shown in FIG. 8D. The waveform of the sum current is illustrated in FIG. 8E.
Therefore, to reduce the rise time of the head magnetizing current i.sub.L flowing through the inductor L, apertures of an input pulse should be corrected. Since recording equalization is possible by generating an input signal of the waveform shown in FIG. 8D in an extra recording equalizer and providing the signal to the recording amplifier, a bit error rate can be improved during playback of a digital signal. Thus, a recording equalization for reducing the rise time of the head current is required for a high-speed, and high-density recording.
Without this recording equalization, a part of the rising portion of the head magnetizing current i.sub.L is lost due to charge and discharge of the stray capacitance C.sub.S, thus being ineffective in magnetizing. As a result, the rise time is increased and the rise characteristic of the current i.sub.L for magnetizing the magnetic tape is lowered, leading to degradation of a high-speed, and high-density recording performance.
FIGS. 9 and 10 illustrate recording amplifiers of a switching type, adopting constant current sources. FIG. 9 shows a single-ended recording amplifier and FIG. 10 shows a push-pull recording amplifier.
A transistor Q.sub.k in the single-ended recording amplifier of FIG. 9 has a current feedback resistor R.sub.k connected to the emitter thereof and thus controls a recording current I.sub.r to be constant. A transistor Q.sub.s functions as a switch for supplying or blocking the recording current I.sub.r according to an input binary coded pulse signal E.sub.i.
If a resistance for the turned-on transistor Q.sub.s is R.sub.ON, a resistance for the turned-off transistor Q.sub.s is R.sub.OFF, and a constant current output impedance is R.sub.S, an equivalent circuit of the recording amplifier shown in FIG. 9 can be obtained as shown in FIG. 11, and the following condition is satisfied in an actual circuit. ##EQU5##
Meanwhile, the waveform of a recording current I.sub.r is illustrated in FIG. 11B.
In FIG. 11B, a rise time constant .tau..sub.r and a rising current I.sub.Rr of I.sub.r are given by ##EQU6## where I.sub.O .perspectiveto.E/R.sub.s.
I.sub.Rr at the start of rising (t ( .tau..sub.r) is given as ##EQU7##
Similarly, a fall time constant I.sub.f and a falling current I.sub.Rf of I.sub.r are expressed as ##EQU8## EQU I.sub.Rf =I.sub.O {1-exp(-t/.tau..sub.f)} (17)
I.sub.Rf at the start of falling (t ( .tau..sub.f) is as follows: EQU I.sub.Rf =E/(N.sup.2 L.sub.h).multidot.t{1-(R.sub.OFF /(N.sup.2 L.sub.h))/2.multidot.t+. . . }
Here, ringings are produced by the stray capacitance C.sub.S and the head inductance N.sup.2 L.sub.h in view of the collector capacitance C.sub.SO of the transistor Q.sub.s shown in FIG. 9. The cycle .tau..sub.rg of these ringings is given in the following equation: ##EQU9##
As noted in equation (18), the resistance N.sup.2 R.sub.h has no significant impact on the rise characteristic of the recording current I.sub.r.
Therefore, since there is a large disparity between the rise time and the fall time, as shown in the current waveform of FIG. 11B, even-numbered high harmonics components are generated in the recording current, and the eye pattern of a reproduction signal is distorted, causing errors.
To prevent generation of these even-numbered high harmonics components, the push-pull recording amplifier of FIG. 10 should be used. An equivalent circuit of the recording amplifier shown in FIG. 10 is illustrated in FIG. 12A and the waveform of the recording current I.sub.r flowing through the head is illustrated in FIG. 12B.
As shown in FIG. 12B, the rise and fall time constants of the recording current I.sub.r are equal and given by EQU .tau..sub.r =.tau..sub.f =N.sup.2 L.sub.h /R.sub.s ( 19)
Further, ringings of the total stray capacitance C.sub.s in view of the parasitic capacitance C.sub.s ' between terminals of the P/T and the R/T and the collector capacities C.sub.SO and C.sub.SO ' of the transistors Q.sub.s and Q.sub.s ' of FIG. 10 are generated to be vertically symmetrical as shown in FIG. 12B.
To enable the head current I.sub.h and the recording current I.sub.r to rapidly rise, the head inductance L.sub.h of the time constants .tau..sub.r and .tau..sub.f of equation (19) should be small, or the output resistance R.sub.s of the constant current source should be large. The head inductance L.sub.h is related with signal reproducing characteristics and, generally, an optimum value is given as the head inductance L.sub.h in terms of a highly efficient playback.
If the output resistance R.sub.s of the constant current source is large, a ringing generation voltage becomes larger and a ringing attenuation becomes smaller, due to the stray capacitance C.sub.s. That is, the larger R.sub.s becomes, the smaller the amplitude and the larger the frequency of the ringing. Thus, R.sub.s is limited to hundreds of ohms. If R.sub.s is 200.OMEGA. and the inductance N.sup.2 L.sub.h at the primary of the R/T is 10 .mu.H, .tau..sup.r =.tau..sub.f =50 ns, not enough for a high-speed recording.
As described above, the prior art recording amplifiers described in connection with FIGS. 1-12 have the following drawbacks.
The A-class recording amplifier of FIG. 1 performs a recording equalization by using a recording equalizer for the input voltage signal E.sub.i received by the transistor Q.sub.a having a constant current control function. Thus, degradation of recording characteristics caused by the parasitic stray capacitance C.sub.S on a recording system can be compensated for, while to operate the linear amplifier, power dissipation is large, and a power transistor is required, entailing the need for a high power voltage. As a result, the recording amplifier can not be compact.
The recording amplifiers of FIGS. 4 and 5 also exhibit the problems of high power dissipation, the need for a power transistor, and inapplicability to a small power-consuming recording.
Therefore, the A- and B-class recording amplifiers can perform recording equalization yet require a linear amplifying function, consuming much power. They cannot satisfy the demands of small size and low-power consumption.
The recording amplifiers of a constant current switching type shown in FIGS. 9 and 10 need a constant current source transistor, not the switching transistors Q.sub.s and Q.sub.s ', as a power transistor. Thus, the recording amplifiers can be small and low power-consuming. However, the input of a recording-equalized voltage signal to the switching transistors Q.sub.s and Q.sub.s ' simply turns the circuits on and off, thus making recording equalization-induced improvement impossible. Further, to reduce the rise time of a recording current, a large band and a high impedance are required as the output characteristics of the constant current source transistor Q.sub.k.